In any surveillance system, in order to properly assess the threat of a potential target, the range to that target is generally required. In surveillance systems employing a passive surveillance scanner, such as an infra-red detector, range information of sufficient accuracy can only be obtained by the use of a separate rangefinder. It is also highly desirable that the use of such a separate rangefinder not disclose the existence of the surveillance to a potential enemy target.
The present invention relates to a laser rangefinder for use in a surveillance system of the kind in which a passive surveillance scanner is rotated in azimuth and, at the same time, is caused to nod in elevation so as to produce an azimuth-elevational scan pattern.
Use of a rangefinder in conjunction with a passive surveillance scanner in a surveillance system gives rise to two competing requirements. On the one hand, if the rangefinder is to provide the same elevation coverage that the surveillance scanner provides, the divergence angle of the beam emitted from the rangefinder should be at least equal to the divergence angle of the scanner which might be, for example, of the order of 45 milliradians. On the other hand, in order to obtain maximum range performance from the rangefinder beam, the divergence angle of the rangefinder beam should be as small as possible, for example, of the order of 3 milliradians.
In order to utilize a narrow divergence rangefinder beam, it is necessary to be able to rapidly deflect the rangefinder beam in elevation within the relatively wider field of view of the surveillance scanner. With such rapid deflection, it would be possible to cause the rangefinder beam to impinge on a target anywhere within the field of view of the surveillance scanner. The requirement for a rapid ranging response time after detection, together with the need to deflect the rangefinder's emitted laser beam, imposes severe mechanical restraints on the nodding reflector and in the rangefinder.
It is therefore an object of the present invention to provide a rangefinder for use in a passive surveillance system, the rangefinder being capable of rapidly deflecting its rangefinder beam relative to the instantaneous position of its reflector by non-mechanical means.